Blender 2.81 toolbar selects tools randomly
&#8679; ShiftSpace by default brings up the tools menu, this can be temperamental in how long it stays visible. Holding space down usually helps to keep it open.

The menu will normally show up positioned so that the last tool used is under the cursor, if the cursor is near the top or bottom of the window it gets moved so that it can be fully shown in the window. This can be part of what leads to it selecting a new (random?) tool and disappearing quickly.

You may find displaying the toolbar on the left side by pressing T to be easier.